1. Field of the Invention
This invention relates generally to a frequency discriminating circuit and more particularly to a frequency discriminating circuit suitable for use with a color demodulator of a color television receiver for receiving a SECAM color television signal.
2. Description of the Prior Art
On manufacturing a frequency discriminating circuit, the center frequency and the linearity of the frequency response characteristic thereof are pre-adjusted to be a predetermined characteristic by suitably selecting values of inductance elements and capacitors used for a double tuned circuit in the frequency discriminating circuit. However, all inductance elements and capacitors do not always have the suitable values in initial, or owing to the circumferential temperature all frequency discriminating circuits are not always of the predetermined characteristic. So, each frequency discriminating circuit must be re-adjusted so as to be of the predetermined characteristic after manufacturing. Both the center frequency and the linearity of the frequency response characteristic of the frequency discriminating circuit (socalled S characteristic) must be of the predetermined characteristic in order that the frequency discriminating circuit is in optimum condition. The center frequency and the linearity are each depending on the resonant frequency and the band pass characteristic of the double tuned circuit in the frequency discriminating circuit. In a conventional frequency discriminating circuit, the inductance values and the coupling factor of primary and secondary windings of a transformer used in a double tuned circuit are adjusted by adjusting the insertion degree of a ferrite core into a bobbin around which the primary and secondary windings of the transformer are wound. By this adjustment, both the resonant frequency and band pass characteristic of the double tuned circuit, that is, the center frequency and the linearity of the S characteristic of the frequency discriminating circuit can be adjusted, but it is impossible to adjust the both independently. That is to say, by this adjustment, one of the center frequency and the linearity can be adjusted in order to be in the optimum condition, but the other of them is not in the optimum condition. In the case that this frequency discriminating circuit is used for a color demodulator of a color television receiver for receiving a SECAM color television signal, as the optimum frequency response characteristic (S-characteristic) can not be obtained, a satisfied image can not be reproduced.